Update string hash algorithm (#93)

As described in https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function
FNV-1 hash lower bits don't have a good entrpy. Use FNV-1a instead, with the
lower bits pruned.

include/frozen/string.h

@@ -84,11 +84,13 @@ template <typename _CharT> struct elsa<basic_string<_CharT>> {
       d = d * 33 + value[i];
     return d;
   }
+  // https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function
+  // With the lowest bits removed, based on experimental setup.
   constexpr std::size_t operator()(basic_string<_CharT> value, std::size_t seed) const {
-    std::size_t d = seed;
+    std::size_t d =  (0x811c9dc5 ^ seed) * 0x01000193;
     for (std::size_t i = 0; i < value.size(); ++i)
-      d = (d * 0x01000193) ^ value[i];
-    return d;
+      d = (d ^ value[i]) * 0x01000193;
+    return d >> 8 ;
   }
 };
 

tests/test_unordered_map.cpp

@@ -1,4 +1,5 @@
 #include <frozen/unordered_map.h>
+#include <frozen/string.h>
 #include <iostream>
 #include <unordered_map>
 
@@ -147,6 +148,26 @@ TEST_CASE("frozen::unordered_map <> frozen::make_unordered_map", "[unordered_map
   }
 }
 
+TEST_CASE("frozen::unordered_map <> std::unordered_map /small", "[unordered_map]") {
+#define INIT_SEQ_SMALL \
+{"0", 0},{"1", 1},{"2", 2},{"3", 3},{"5", 4},{"8", 5},{"9", 6},{"A", 7},{"W", 8},{"X", 9},{"r", 10},{"y", 11},{"BF", 12},{"AP", 13}
+
+  const std::unordered_map<frozen::string, int> std_map = { INIT_SEQ_SMALL };
+  constexpr frozen::unordered_map<frozen::string, int, 14> frozen_map = { INIT_SEQ_SMALL };
+  SECTION("checking size and content") {
+    REQUIRE(std_map.size() == frozen_map.size());
+    for (auto v : std_map)
+      REQUIRE(frozen_map.count(std::get<0>(v)));
+    for (auto v : frozen_map)
+      REQUIRE(std_map.count(std::get<0>(v)));
+  }
+
+  static_assert(std::is_same<typename decltype(std_map)::key_type,
+                             typename decltype(frozen_map)::key_type>::value, "");
+  static_assert(std::is_same<typename decltype(std_map)::mapped_type,
+                             typename decltype(frozen_map)::mapped_type>::value, "");
+}
+
 TEST_CASE("frozen::unordered_map constexpr", "[unordered_map]") {
   constexpr frozen::unordered_map<unsigned, unsigned, 2> ce = {{3,4}, {11,12}};
   static_assert(ce.begin() +2 == ce.end(), "");